Sensor development is a fertile field of investigation, having crucial importance in both economic prosperity and national security. For example, non-imaging sensors are used to monitor such parameters as temperature, acceleration, pressure, position, relative humidity, voltage and current, which are particularly helpful in industrial applications such as automobile engine control systems and flexible computer integrated manufacturing. Sensors and signal processing allow automated systems to interact with the external world, and are important components of vital systems in the fields of defense, aerospace, bioprocessing, human health care, manufacturing, pollution control, transportation and telecommunications.
Sensors also play an important role in analyte detection. Analyte detection can serve many purposes. The identification of analytes is useful in myriad applications across many industries. Reliable, efficient, inexpensive, and fast recognition of chemical and biological analytes is an important goal in many applications and industries.
One of the more recent concerns and applications for chemical and biological sensors relates to human safety. Detecting hazardous chemical and biological agents is useful in manufacturing facilities and health facilities, for example. The risk of chemical and biological terrorism provides a role for chemical and biological sensors, as well. Prominent biological agents for bioterrorism, threats of bioterrorism, and biological warfare, such as Bacillus anthracis spores, botulinum toxin, smallpox, and plague, are reasonably easy to prepare and disperse, and have the potential to inflict horrific injury and death upon a population.
Biological and chemical sensors must operate in difficult environments. A typical analyte detection environment can include many other agents that can make detection of the analyte(s) of interest difficult. Example agents that can thwart detection include various gaseous chemical compounds, microorganisms, and particulate matter. For example, a sensor for detecting chemical and bioterrorism agents can operate in an environment with hundreds of ambient chemicals, many microorganisms, and many other particles.
Also, it is useful for a signal sensor to detect multiple analytes and be able to provide a definite indication of both. However, many sensors are chemically prepared to attract a single type of analyte. In environments where there are multiple analytes of concern, multiple single analyte sensors are required.
Detection of biological agents in particular is also difficult owing to the relatively large size of most biological agents. For example, even one of the smallest biological agents, aflatoxin, weighs approximately 300 daltons (Da), whereas bacterial spores such as anthrax weigh approximately 667 million times more than aflatoxin. Biological agents include as many as 20 amino acids, and cells are complicated in that they include lipids in their membranes and other unique molecules as well.
In addition to hazardous material detection, drug control is another area that can benefit from a reliable chemical sensor. The Committee on Science of the 109th Congress has urged research on detection, standardization, and remediation of methamphetamine (Meth). Their report indicates that Meth labs have grown from 218 in 1993 to 15,000 in 2004. This report brings out the urgent need for new Meth detection technologies with emphasis on field test kits and site detection. Testing of drugs and drug materials in a container is a major problem encountered by law enforcement officials. Limited amount of inspections of these containers are carried out manually and it is estimated that the U.S. now inspects 4 percent of the 6 million shipments that arrive at more than 100 ports, double the percentage before the September 11 attacks in 2001 [Edward J. Staples and Shekar Viswanathan, Paper read at the 7th World Congress of Chemical Engineering, 10-14 Jul., 2005, Glasgow, Scotland]. Several government organizations such as the National Institute of Justice, Federal Bureau of Prisons, Department of Defense, Counter drug Technology Development Program Office, and others are working on developing sensitive sensors.
Existing sensors exhibit inherent drawbacks. One method for sensing biological agents includes traditional substrates, such as silicon or diamond-like carbon, for binding various enzymes, DNA, and proteins to a film, such as a diamond film. While this method is selective, it requires different enzymes for different purposes, and as such, each sensor must be designed for a specific agent. Moreover, this conventional method is an unduly slow process, is expensive, is limited to the detection of a single agent, and can only be used once. Additionally, conventional methods are often non-specific, and the equipment involved in conventional processes is often bulky or remote from the location to be tested.
Another drawback of conventional sensors and conventional sensing methods is that diffusion is relied upon to adhere a biological or chemical agent to a surface of the sensor. Thus, these conventional sensors and sensing methods are reliant upon a random process that results in a net loss of sensitivity by the sensor.
There are also several methods being investigated for biological agent detection. The most common method is the antibody-based detection and identification systems that can discriminate between biological agents on the three-dimensional structure of the component molecules of the agents. A second method is the Polymerase Chain Reaction (PCR) and gene probe based detection technique. The PCR uses the nucleic acid sequences of genes to differentiate the bioagent. Some biological molecules have unique ultraviolet light absorption properties. These molecules will then fluoresce at a specific wavelength that is detectable.
One approach tries to exploit these UV absorption and fluorescence properties. There are other approaches being investigated, including, for example, infrared backscatter for aerosol detection. This method can size a small particle but cannot distinguish it from a bio- or non-bio-aerosol. Man-made biological aerosols can have a unique shape and/or a unique size. It may be possible to develop methods to determine the size and shape of the aerosol. Biomolecules may have significant hydrophobicity and this may be used to detect bioagents. It may also be feasible to use a network of sensors to detect a two-dimensional pattern unique to a biological weapons attack. The primary problem which is shared by each of the above technologies is the time required for unambiguous detection. This is true whether or not the operational scenario is based on detect-to-warn or detect-to-treat objectives.